Investigation of Polar Mesospheric Summer Echoes observed with the EISCAT VHF radar

Abstract

During the summer months at high and mid-latitudes, VHF and UHF radars can detect strong radar echoes, known as Polar Mesospheric Summer Echoes (PMSE), at altitudes between 80 and 90 km. The formation of PMSE is closely linked to the presence of ice particles, turbulence, and irregularities in the electron density. Current theories suggest that neutral turbulence causes the spatial structuring of charged ice particles, which in turn reduces electron diffusion and allows for the observation of spatial structures in electron density at half the radar wavelength.

This thesis investigates the properties of PMSE, focusing on their spectral shape, morphology, and layering based on 141 hours of observations by the EISCAT VHF radar from 2010 to 2021. The EISCAT Real-Time Graph (RTG) software was employed to obtain the power spectra, from which spectral width, Doppler shift, and received echo strength were derived by fitting a Gaussian function. The obtained parameters were used to classify PMSE data.

The analysis considers the influence of geomagnetic activity and potential gravity wave activity on PMSE. Further investigations focused on the characteristics of multilayer systems and the spectral parameter as functions of altitude.

The results indicate that signal strength is influenced by various factors, including temperature profile, aerosol size, ionization rate, number of layers, and electron density. A direct response in signal strength is observed during particle precipitation. The distribution of signal strength for all PMSE data shows a slight decrease with altitude, suggesting a possible relationship with larger-sized aerosols. Notably, a local minimum in mean signal strength occurs around 86 km, which is close to the average altitude of all observed PMSE.

Regarding multilayer systems, the integral of the power spectrum tends to decrease as the number of layers increases, while the spectral width shows a slight increase with the number of layers. The spectral width of layers within a multilayer system appears to have approximately the same value.

The changes in spectral width present a more complex scenario. Enhanced spectral width values are observed during events of particle precipitation, alongside changes in the morphology of PMSE and instances of wave signatures in the Doppler shift, which may indicate enhanced turbulence. However, these changes could also be closely related to the influence of the incoherent scattering spectrum in the signal processing. Additionally, a slight trend of increasing spectral width with altitude is noted across all PMSE data.